In recent years, a demand for quality of a silicon single crystal produced by Czochralski method (hereafter abbreviated to CZ method) used as a substrate has been increasing due to decrease in size of a device resulting from increased degree of integration of a semiconductor circuit. There exist defects introduced during growth of a single crystal called grown-in defects such as FPD, LSTD and COP, which degrade oxide dielectric breakdown voltage characteristics and device characteristics. It has been considered that it is important to decrease a density and a size thereof.
For explanation of the above-mentioned defects, first are described general matters known as for factors which determine a concentration of point defects called vacancy (hereinafter occasionally referred to as V) and a concentration of point defects called interstitial silicon (Interstitial-Si, hereinafter occasionally referred to as I).
In a silicon single crystal, V region refers to a region which contains a large amount of vacancies, i.e., depressions, pits, or the like caused by lack of silicon atoms; and an I region refers to a region which contains a large amount of dislocations caused by existence of excess silicon atoms or a large amount of clusters of excess silicon atoms. Further, between the V region and the I region, there exists a neutral (hereinafter occasionally referred to as N) region which contains no (or little) surplus or no (or little) shortage of silicon atoms. Recent studies have revealed that the above-mentioned grown-in defects (such as FPDs, LSTDs and COPs) are generated only when V or I are present in a supersaturated state and that even when some atoms are unevenly distributed, they do not appear as a defect so long as V and I do not exceed the saturation level.
It has been confirmed that a concentration of each of these point defects depends on the relation between a pulling rate (growth rate) of the crystal in CZ method and a temperature gradient G near a solid-liquid interface of the crystal, and that another type of defect called oxidation-induced stacking fault (hereinafter occasionally referred to as OSF) is present in a ring-shape distribution near a boundary between V region and I region, when the cross section perpendicular to the axis of crystal growth is observed.
When a crystal is pulled through use of a CZ pulling apparatus with a furnace structure having a large temperature gradient G near a solid-liquid interface of the crystal (hereinafter occasionally referred to as hot zone: HZ) with growth rates varying from high speed to a low speed along the crystal axis, these defects introduced during the crystal growth exist as in a distribution chart of defects shown in FIG. 7.
These defects introduced during the crystal growth can be classified as follows. When the growth rate is relatively high, for example, about 0.6 mm/min or higher, grown-in defects such as FPDs, LSTDs and COPs which are considered to be generated due to voids consisting of aggregated vacancy-type point defects are present at a high density over the entire radial cross section of the crystal. The region where these defects are present is called V region (See FIG. 7, line (A)). When a growth rate is not higher than 0.6 mm/min, the OSF ring is generated with decrease of the growth rate, from a circumferential portion of the crystal. If the growth rate is decreased further, a diameter of the ring shrinks, and defects of L/D (large dislocation, abbreviation of interstitial dislocation loop, LSEPD, LFPD or the like) which are considered to be generated due to dislocation loop are present at a low density outside the ring. The region where these defects are present is called I region (hereinafter occasionally referred to as L/D region). Furthermore, when the growth rate is decreased to about 0.4 mm/min or less, the OSF ring shrinks to the center of a wafer and disappears, so that the I region spreads over the entire plane of the wafer (See FIG. 7, line (C)).
Furthermore, there has been recently found existence of a region, called N region, where there is located between the V region and the I region and outside the OSF ring, and where there exists neither defects due to vacancies such as FPDs, LSTDs and COPs nor defects due to a dislocation loop such as LSEPDs and LFPDs. It has been reported that located outside the OSF ring is the region where substantially no oxygen precipitation occurs when a single crystal is subjected to a heat treatment for oxygen precipitation and the contrast due to precipitates is observed through use of an X-ray beam or the like, and that the region is on an I region side and is not rich enough to cause formation of LSEPDs and LFPDs (See FIG. 7, line (B)).
Since these N regions exist inclining from growth axis when a growth rate is lowered in the case of general methods, they exist only in a part of a plane of the wafer.
According to the Voronkov theory (V. V. Voronkov; Journal of Crystal Growth, 59 (1982) 625-643), it is proposed as for N region that a total concentration of a point defect is defined by a parameter called V/G which is the ratio of a pulling rate (V) and a temperature gradient (G) along the axis direction in the crystal solid-liquid interface. Considering the above theory, only a crystal wherein V region exists at a center and I region exist around it over N region can be obtained at a certain pulling rate, since a pulling rate is constant in a plane and G is distributed in the plane.
Recently, it is proposed that the N-region which could exist only slantwise is enlarged by improving distribution of G within a plane. For example, when a crystal is pulled with decreasing a pulling rate V gradually, the crystal in which the N region spread horizontally over the whole plane can be obtained at a certain pulling rate. Enlargement of the crystal having N region spreading horizontally over the whole plane into a direction of length can be achieved to some extent by maintaining the pulling rate at which the N region spreads horizontally. By controlling a pulling rate so that V/G value may be constant with considering that G is varied with growth of the crystal and calibrating it, the crystal having the N region over the whole plane can be enlarged into a direction of growth to some extent.
The N region can be classified into Nv region (the region where a lot of vacancies exist) adjacent to the outside of OSF ring and Ni region (the region where a lot of interstitial silicons exist) adjacent to I region. It has been found that a lot of oxide precipitates are generated in the Nv region when thermal oxidation treatment is carried out, and that there is almost no oxide precipitates are generated in the Ni region.
However, it has been found that an oxide-film defects may occur remarkably, even if it is a crystal such as the above-mentioned single wherein the N region occupies the whole plane, an OSF ring is not generated when thermal oxidation treatment is carried out, and FPD and L/D do not exist in the whole plane. This may be a cause of degrading electrical characteristics such as oxide dielectric breakdown voltage characteristics. Accordingly, the fact that the N region occupies whole plane as in a conventional crystal is not enough, and the further improvement has been desired.